(1) The previous birth? Siddhi are you sure you are not mixing perhaps Hindu influences of reincarnation with Christianity? There is no reincarnation in Christianity. The only new birth we have is our new birth into our new life in Jesus Christ our Lord!! You are not in your present situation because of some sin of a past birth.
(2) Perhaps it would be good for you to think of the positives of your current situation. For example, you have come to the saving knowledge of Jesus Christ because of your current situation. Think about it, your current situation caused you to seek to find the answers for yourself of who the real God really was. If you had all the things in your life other young people seem to have, everything was going well for you say, everything was perfect in your life, perhaps you wouldn't have felt the need to cry out, to seek out who God really was. And then what would have happened? You would have been left in the dark never coming to know Jesus Christ. Your earthly life may have been good, but your spiritual life would have suffered. No, rather it was your weakness that led you to seek the Lord. So I ask you if you had to do it all over again would you rather have had the perfect earthly life and died one day in spiritual blindness or would you rather have not had the perfect earthly life like you are experiencing, but it leading you as you have to find the truth?
(3) You are right there is much sin and darkness in the world Siddhi. And it may get even darker in time. But here is the thing, sometimes just as in our individual lives, the Lord also has to also take the world through processes and pressures and yes even dark times in order to get to Him. The world will one day be made to see the answer is not in itself. The world since the days of Adam have desired to be like God deciding for themselves what is good and evil, and the hole they have dug only gets deeper and deeper. We just have to be brave soldiers of Christ in the midst of these processes and bear the day and the times the Lord has appointed us to live, pray for others, trust in the Lord's processes and all He decides He needs to bring the world through in order to bring all of creation back to what it was always intended to be. There is nothing we can do but that and desire to be the light of Him in all that we do.
(4) As to depression and pills. I personally don't think your answer is pills Siddhi. I think you answer is acceptance and trust. In everything listed above it comes down in all of these things you are struggling with to two things. Acceptance and Trust. Your circumstances are what they are. The world is what it is. Yours and mine and everyone's spiritual condition is what it is. Who of any of us on our own merits can be saved? So what are you going to do with all of that. The only true way to peace is to put it in His hands, Accept it just the way it is, and Trust Him and the processes He uses to work it out. If you come into that sort of Acceptance and Trust I believe you come into peace. That is your answer if you ask me.
Here is a scripture for you as it relates to all of this. It came in a period in Israel's history when they were troubled and frightened by many things. They could not come to simple rest, acceptance and trust in God. They looked everywhere except Him. But He told them in that day and I believe He tells you the same word.
In returning and rest you shall be saved; in quietness and in trust shall be your strength
(Isaiah 30:15)